You should still be able to train in many abilities since you are still members at the appropriate guilds. Skill points you can use anyway you like providing the skill hasn't scrolled off the screen (a design problem but not one that troubles most players).
The limitation to changing class seems to be what level spells they will still be able to learn. I changed one character from priest to warlock or some such thing but the result seemed to be that she was unable to learn any spells higher in her previous profession than where she was when she changed class. I'm replaying the game with the same characters, having reset the quests, and am trying to bump her up in those spells now. I might have waited longer to change her the first time through.